This invention relates generally to the field of duplex communications, and more particularly to systems wherein a master unit transmits to and receives from a multiplicity of subsidiary units over a single channel.
There are many types of systems wherein one master unit communicates with a number of subsidiary units utilizing a two-way flow of data over a single communication channel. There are paging systems using RF transmission wherein each unit has an assigned address and the master unit transmits a given address, encoded in some fashion such as a sequence of tones modulated onto a carrier as exemplified in U.S. Pat. No. 3,766,523, or as digital "words" in a bit stream as in in U.S. Pat. No. 3,855,576, both patents being assigned to the same assignee as is the present invention. The latter patent will be referred to again hereinbelow. In the former patent, the pager unit having the assigned address decodes it upon reception and in one of a number of ways alerts the pager user that he has been paged.
Another application requiring a two-way flow of data involves a large computer, a control unit and multiple satellite CRT terminals, all inter-connected by cables. Such a system might be used as on a production line where the assembled products; e.g., automobiles, may require special components for certain units going down the line. Information relative to each individual unit's requirements is stored in the master computer. An operator along the line can query the computer via one of the CRT terminals as to any special components. The computer, of course, will reply almost instantaneously, but the operator must then return to the production line, where the unit in question has been moving along. Such systems typically poll each CRT terminal in sequence, wait for an answer, and then move on at the end of the answer or after a set period such as three seconds.
Another system, used as in inventory applications, utilizes detached portable units including a data entry terminal and data storage means. The user enters inventory data as he or she moves along the aisles of a warehouse, by keyboard and/or light pen. Then, at the end of a given time period, perhaps several hours, the portable unit is brought to a master computer terminal and plugged in for transfer of the stored data into the master computer. It is obvious that in the two latter systems, a detached or completely portable two-way terminal in constant touch with the computer would be preferred, with some means for controlling the flow of data between the portable units and the master or host computer for maximum channel efficiency.